The earliest retroreflective sheeting had an exposed-lens construction, but its reflex-reflective light was blanked out when the lenticular surface of the exposed lenses was covered with water. This problem was answered by enclosed-lens retroreflective sheeting in which, as first taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), the lenses were embedded within the sheeting which had a flat, transparent cover film. This allowed incident light rays to be focused onto the specularly reflective layer irrespective of whether the front of the sheeting was wet or dry. The above-cited McKenzie patent solved the same problem in a different way, namely, by modifying retroreflective sheeting of the exposed-lens type wherein lenses are partially embedded in a binder layer. In the McKenzie patent, the exposed lenses are protected by a cover film to which the binder layer is sealed along a network of interconnecting lines, thus forming a plurality of hermetically sealed cells within which the lenses are encapsulated and have an air interface. Such exposed-lens sheeting is called "encapsulated-lens retroreflective sheeting".
In the method taught in the McKenzie patent for making encapsulated-lens retroreflective sheeting: (1) substantially a monolayer of lenses such as glass microspheres is embedded into a carrier web to a depth not exceeding 50% of the diameter of each microsphere, (2) specularly reflecting material is deposited over the lens-bearing surface of the carrier web, (3) a solution of binder material is applied over the specularly reflecting deposit, (4) after drying the binder, the carrier web is stripped off, (5) a cover film is laid over the exposed microspheres, and (6) heat and pressure are applied along a network of interconnecting lines to soften the binder material to allow it to flow around the microspheres and into contact with the cover film, thus forming the aforementioned hermetically sealed cells. It is believed that in the manufacture of all such encapsulated sheeting, the binder material includes a white pigment such as TiO.sub.2 to give the sheeting a whiter color as well as a cleaner color in any area to which another color has been applied by silk screening. The whiteness of the sheeting is enhanced if the specularly reflective material, usually aluminum, between the microspheres is carried away by the carrier web. To this end, the binder material may include a release agent such as stearic acid, but a release agent tends to interfere with the bonds between the binder layer and both the glass microspheres and the cover film. Better structural integrity is needed for uses in which the retroreflective sheeting is to be subjected to flexing or abrasion, as in roadlane markers, or impact, as in traffic cones.
Another method for making encapsulated-lens retroreflective sheeting is taught in U.S. Pat. No. 4,075,049 (Wood). The first two steps of the Wood method are substantially the same as the first two steps of the McKenzie method, but in the third step of the Wood patent, some of the lenses 11a are forced into the carrier web 13 along a network of grid lines 18 as illustrated in FIGS. 3 and 4. Then a cast binder layer 23 is applied over the undisplaced lenses 11 such that some of the binder layer flows into the grid pattern between the undisplaced lenses (FIG. 5A). After the carrier web and the displaced spheres have been stripped off (FIG. 6) comparably to step (4) of the McKenzie patent, a flat, transparent cover film 27 is adhered to the binder layer at the grid pattern, between which the lenses are encapsulated and have an air interface 28 as shown in FIG. 7.